Plastic film is generally produced by extruding plastic material such as polyethylene resin in a process known as the “blown film process.” FIG. 1 illustrates a system 100 for producing plastic film utilizing the blown film process. Polyethylene resin, in the form of pellets, is fed into a hopper 104 of an extruder 108. The pellets are heated in the extruder 108 until they are molten. The molten material is fed under pressure into an extrusion die 112 having an annular outlet 116. As the molten material is extruded from the extrusion die 112, a plastic film tube or bubble 120 emerges. The terms “tube” and “bubble” are used interchangeably hereinafter.
The plastic film tube 120 is blown or expanded to a larger diameter by feeding air into its interior, thereby forming a thin film. By controlling the air pressure in the interior of the plastic film tube 120, the cooling rate of an air ring 132, the speed of an upper nip 122, and amount of resin extruded from the extrusion die 112, the plastic film tube 120 is blown to a desired thickness.
The plastic film tube 120 solidifies from a molten mass to a solid as it expands to its final diameter and travels in a vertically extending tube path. The lower portion of the plastic film tube 120 below the line 124 is molten while the upper portion of the plastic film tube 120 above the line 124 is solidified. The plastic film tube 120 travels within a bubble cage or frame 128 as it travels in a vertically extending tube path.
The plastic film tube 120 is generally cooled externally by one or more external air rings 132 that surround the tube. The air rings 132 direct a flow of cooling air onto the outer surface of the plastic film tube 120.
The plastic film tube 120 is additionally cooled internally by an internal bubble cooling (IBC) assembly 136. The IBC assembly 136 is configured to direct cool inlet air onto the inner surface of the plastic film tube 120. The IBC assembly 136 is coupled to an air inlet passage 140, which carries cool inlet air (i.e., cooling air) to the interior of the plastic film tube 120 through the extrusion die 112. Heated air from the interior of the plastic film tube 120 is collected by an exhaust air shaft 144, and the heated exhaust air is removed by an outlet passage 148 coupled to the exhaust air shaft 144.
The cooling of the plastic film tube 120 is a critical factor in the production of plastic film from plastic materials such as from polyethylene resin. The cooling rate influences the production capacity and the film properties. A controlled high cooling rate allows a high extrusion rate, resulting in increased plastic film production while maintaining desired film properties.
FIG. 2 illustrates a cross section of an apparatus 200 for producing plastic film tube in a blown film process. The apparatus includes an IBC assembly 204 having a plurality of air dispensing devices 208 stacked on top of an extrusion die 212. The air dispensing devices 208 include slits 216 for directing cooling air onto the inner surface of a plastic film tube 220. As shown by the arrows, the cooling air out of the slits 216 is directed onto the inner surface of the plastic film tube 220. The angle from horizontal at which the cooling air impacts the plastic film tube depends on the geometry of the slits or slots 216 (horizontal, angled upwards, or angled downwards) and the angle of the plastic film tube from vertical. The cooling air flow impacts the plastic tube 220 as a two-axis air flow: the air flows initially radially out of the slots at an angle to the horizontal and then travels in a generally upward direction due to the upward motion of the tube 220 and suction of the exhaust shaft 228. The cooling air is carried to the interior of the plastic film tube 220 by an inlet passage 224. An exhaust shaft 228 collects heated exhaust air from the interior of the plastic film tube 220, and the exhaust air is removed via an outlet passage 232 coupled to the exhaust shaft 228.
The air dispensing devices 208 regulate the velocity, volume and the direction of the air directed onto the inner surface of the tube or bubble. Increasing the velocity or volume of the air directed onto the inner surface of the tube increases the rate of cooling, which allows increased production of the film. However, when air is directed with increased velocity or volume onto the inner surface, the tube is often cooled non-uniformly as certain areas of the tube receives more cooling air than other areas, which results in non-uniform film thickness. Also, directing air onto the inner surface of the plastic film tube 220 sometimes causes the plastic film tube 220 to become unstable, which may result in the collapse of the plastic film tube 220. If the plastic film tube 220 collapses, the process must be shut down in order to remove molten material from atop the extrusion die 212. Thus, the instability of the plastic film tube 220 caused by the direction of the air flow may result in inefficiency in the production of the plastic film tube 220.
Accordingly, there is a need for an apparatus and method for cooling the plastic film tube without causing instability. There is a need for an apparatus and method for cooling the plastic film tube uniformly, thus preventing non-uniform film thickness. There is a need for an apparatus and method for efficiently cooling the plastic film tube without the foregoing disadvantages.